Sea lampreys, Petromyzon marinus, belong to a group of fish called Agnathans, which lack a jaw. Although classified in the subphylum Vertebrata, this species lacks vertebrae, and their entire skeleton is cartilaginous. Commonly known for their smooth, scaleless physique and long cylindrical bodies, they are often misidentified as eels. Sea lampreys also lack swim bladders and a lateral line system. Members of this species have a visible eye spot located on each side of their head behind a single nostril and above a set of seven gill openings. Their mouth takes on an oval shape while attached to their host, but once opened it becomes larger than the head and pharynx together. Inside the oval-shaped mouth are numerous rows of large teeth pointing inward. Sea lampreys have two dorsal fins but lack any paired fins. When spawning occurs, males develop a distinct ridge along their back and females develop a pronounced fold of skin behind their vent.
Sea lampreys are the largest and most aggressive species of lamprey, ranging from 15.2 to 30 cm in length as juveniles and 30 to 100 cm in length as adults. Adults can weigh up to 2.5 kg. Besides length, there are several key differences between adult and young sea lampreys. Color is often a good indicator of age; larvae generally are dark, greenish brown with a light grey underbelly, while adults are brownish grey and tend to lighten in color when about to spawn. Another key difference involves the dorsal fins; while separate in young lampreys, the dorsal fins migrate closer together as sea lampreys reach adulthood.
Range mass: 1 to 2.5 kg.
Range length: 15.2 to 30 cm.
Other Physical Features: bilateral symmetry ; polymorphic
Sexual Dimorphism: sexes shaped differently